You can contribute to two Wikimedia projects: Wikipedia and Wikimedia Commons.
Did you know that when you see an image on Wikipedia, it is not stored on the encyclopaedia? Wikimedia Commons is an on-line media library holding all the images that it is possible to display on Wikipedia or another site. It contains over ten million files (July 2011) and all are freely usable by anybody, provided you cite the author.
Anybody can supply Wikimedia Commons with images, diagrams, photos, videos, etc.